The present invention relates to a coated optical fiber comprising a cured coating prepared from a radiation curable composition having improved cure speed without deteriorating the yellowing performance of the cured coating. In particular, the radiation-curable composition of the present invention is a liquid curable composition that can be formulated for use in a wide variety of applications including, for example, coatings and/or binders. In particular, these curable compositions offer relatively fast cure speeds which offer advantages in many applications such as in the production of fiber optics. In the latter, production speeds make it desirable to utilize primary coatings, secondary coatings, matrix materials, bundling materials (all of which can be transparant or colored) and/or inks that can be cured rapidly. Moroever, the present invention relates to a radiation curable composition, to a cured coating and to a method of increasing the cure speed of a radiation curable composition.
In the production of optical fibers, a resin coating is applied immediately after drawing of the glass fibers for protection and reinforcement of the glass fiber. Generally, two coatings are applied, a soft primary coating layer of a flexible resin (low modulus and low Tg) which is coated directly on the glass surface and a secondary coating layer of a rigid resin (higher modulus and higher Tg) which is provided over the primary coating layer. Often, the fibers for identification purposes will be further coated with an ink, which is a curable resin comprising a colorant (such as a pigment and/or a dye), or the secondary coating may be a colored secondary coating (i.e, comprise a colorant).
Several coated (and optionally inked) optical fibers can be bundled together to form a so-called optical fiber ribbon, e.g., four or eight coated (and optionally inked) optical fibers are arranged on a plane and secured with a binder to produce a ribbon structure having a rectangular cross section. Said binder material for binding several optical fibers to produce the optical fiber ribbon structure is called a ribbon matrix material. In addition, a material for the further binding of several optical fiber ribbons to produce multi-core optical fiber ribbons is called a bundling material.
One of the most important characteristics required nowadays for curable resins used as coating materials (for protective or identification purposes) for optical fibers is to have a cure speed that is sufficiently high to be applicable at the currently used increasing optical fiber drawing speeds while still being cured thoroughly. Moreover, this improved cure speed should be obtained without sacrificing the chemical and mechanical properties of the cured coating. At present, in the production of optical fibers and optical fiber assemblies, one of the limitations on how fast the production line can be operated is the cure speed of the coatings and/or binders. Accordingly, it is desirable to develop coatings and/or binders with faster cure speed.
Besides having a high cure speed, the coating must also fulfill many other requirements, in particular: exhibiting very little physical change over a long period of time and also over wide temperature ranges; having acceptable resistance to heat and light (and thus, showing acceptable aging properties such as a low degree of yellowing), to hydrolysis, to oil, and to chemicals such as acids and alkalis; absorbing only a relatively small amount of moisture and water; producing little hydrogen gas which adversely affects optical fibers; and the like.
Resins that cure on exposure to radiation such as ultraviolet radiation are favored in the industry, due to their fast cure, enabling the coated fiber to be produced at high speed. In many of these radiation curable resin compositions, use is made of urethane oligomers having reactive terminal groups (such as an acrylate or methacrylate functionality, herein referred to as (meth)acrylate functionality) and a polymer backbone. Generally, these compositions may further comprise reactive diluents, photoinitiators, and optionally suitable additives.
From WO 98/00351 it is known to use a mixture of two hydrogen abstraction free-radical photoinitiators and one xcex1-cleavage, homolytic free-radical photoinitiator, in particular a mixture of 1.5 wt. % benzophenone, 0.5 wt. % benzil with sensitizer diethyl amine and 0.7 wt. % Irgacure 651.
Photoinitiator packages are commercially available e.g. Esacure KTO 46 (consisting of a benzophenone derivative, a phosphine oxide type photoinitiator and an oligomeric xcex1-hydroxy acetophenone) and Esacure KIP 100 F that is a mixture of 70% of an oligomeric xcex1-hydroxy acetophenone and 30% of dimethyl hydroxy acetophenone.
So far, however, the known radiation-curable compositions also if they comprise a combination of photoinitiators in varying amounts do not provide satisfying properties with respect to cure speed in combination with mechanical and aging properties of the resulting cured coating. Moreover, using a high amount of photoinitiator often results in drawbacks such as a relatively high amount of extractables, yellowing of the cured coating over time, and other undesired properties.
An object of the present invention is to provide a coated optical fiber comprising a cured coating prepared from a liquid curable resin composition which exhibits a fast cure speed without impairing mechanical properties and aging characteristics of the coating. In particular, a coated optical fiber comprising a cured coating having a relatively low yellowing is desired. A further object of the present invention is to provide a radiation curable composition having improved cure speed and a method of increasing the cure speed of a radiation curable composition.
The present invention provides a coated optical fiber comprising a glass optical fiber with a single protective coating or a combination of an inner and an outer primary coating applied thereon and optionally with a colored coating subsequently applied thereon wherein the inner primary coating or at least a portion of the single coating is prepared from a radiation curable composition which when cured as a capillary film with a 100 W medium pressure mercury lamp has a,percentage reacted acrylate unsaturation of at least about 54% after exposure to a dose of about 4.4 mJ/cm2 or wherein the outer primary coating is prepared from a radiation curable composition which when cured as a capillary film with a 100 W medium pressure mercury lamp has a percentage reacted acrylate unsaturation of at least about 56% after exposure to a dose of about 4.4 mJ/cm2.
According to the present invention, at least one of said coatings is prepared from a radiation curable composition which when cured as a capillary film with a 100 W medium pressure mercury lamp has a percentage reacted acrylate unsaturation of at least about 56% after exposure to a dose of about 4.4 mJ/cm2.
Further, the present invention provides a radiation curable composition comprising
(A) an oligomer,
(B) a reactive diluent, and
(C) a photoinitiator package of at least two free radical photoinitiators having an overall absorption spectrum in methanol which is the sum of the absorption spectra of each individual photoinitiator wherein said overall absorption spectrum has a minimum value of a molar extinction coefficient xcex5 in a range between 280 nm (xcex1) and 320 nm (xcex2) of at least about 600 lmolxe2x88x921 cmxe2x88x921.
The invention also provides a radiation curable composition comprising
(A) an oligomer,
(B) a reactive diluent, and
(C) at least three free radical photoinitiators wherein
(i) at least one of the photoinitiators has an absorption spectrum in acetonitrile having a difference between two absorption maxima xcex94xcexmax(lij)=(xcexmax)ljxe2x88x92(xcexmax)li in the range between 240 and 360 nm of at least about 15 nm, and wherein
(ii) considering at least two of the photoinitiators (1 and 2), the difference between the absorption maximum (xcexmax)1 of the absorption spectrum in acetonitrile of photoinitiator 1 and the absorption maximum (xcexmax)2 of the absorption spectrum in acetonitrile of photoinitiator 2 xcex94xcexmax(12)=(xcexmax)2xe2x88x92(xcexmax)1 in the range between 280 and 320 nm is at least about 5 nm.
Furthermore, the invention provides a radiation curable coating composition for optical glass fiber comprising
(A) an oligomer,
(B) a reactive diluent, and
(C) at least four different photoinitiators (C1), (C2), (C3), and (C4).
Suprisingly, the coated optical fiber of the present invention has good properties even if produced at extremely high line speeds whereas other coated optical fibers have inferior quality. In particular, the radiation curable composition according to the present invention overcomes all the drawbacks of the prior art composition by choosing combinations and amounts of photoinitiators in such a way that an improved cure speed can be obtained without substantially deteriorating mechanical properties and yellowing characteristics (including initial yellowing and yellowing upon aging).
The composition is applied to a substrate and cured. The compositions of the invention are preferably designed for use as a colored or uncolored optical fiber single protective coating, primary (or inner primary) coating, secondary (or outer primary) coating or related optical fiber protective materials such as matrix or bundling materials. Such optical fiber coatings have their own set of unique performance requirements, which distinguish them from conventional applications.
Finally, the present invention provides a cured coating and a method for increasing the cure speed of a radiation curable composition.